The present disclosure relates to an optical modulator, an imaging device and a display apparatus, and in particular, an optical modulator using graphene, an imaging device and a display apparatus using the optical modulator.
It is necessary for an imaging device to control an incident light intensity to a predetermined light intensity to enlarge a dynamic range. Moreover, it is necessary for a display apparatus such as a liquid crystal display or an organic electroluminescence display (organic EL display) to control light intensity used in displaying to a predetermined value to express a gradation. In addition, when such light intensity control is performed, it is necessary to change the light intensity at a predetermined speed.
In the related art, as a method of electrically controlling light intensity, for example, there are a method of controlling an orientation of liquid crystal molecules and a method of using an electrochromic material (tungsten oxide, viologen and the like) in which a reversible change in a molecule structure is used (for example, refer to PTL 1, PTL 2 and NPL 1).
However, since response speed is not sufficiently fast due to the orientation operation of liquid crystal molecules, and there is a large amount of loss in transmitted light intensity by the liquid crystal molecules, the method of controlling an orientation of liquid crystal molecules does not have a sufficient transmittance in a transparent mode. Moreover, there is a problem that an orientation portion and an electrode portion are enlarged in a liquid crystal unit. In addition, the response speed achieved by the method using an electrochromic material, the response time is in the order of millisecond, is low in comparison with that achieved by the method using the orientation of liquid crystal molecules, and there is a limitation in a wavelength range in which light intensity can be adjusted. Additionally, since it is difficult to reduce a light intensity control mechanism in size by using these methods, for example, light intensity control for each pixel of a solid state imaging device is difficult.
On the other hand, graphene has been paid significant attention to in recent years. Graphene, which has a thickness of the order of a monoatomic layer only made of carbon atoms, is a two-dimensional material, and is a zero-gap semiconductor which is different from general semiconductors in that it has a linear band structure. Dopants are absorbed into the surface of the graphene, that is, chemical doping is performed to induce carriers so that the Fermi level of the graphene can be shifted. When the energy of an amount of the shift in the Fermi level is set to delta EF, the light absorption of the energy of equal to or less than 2|delta EF| is prohibited from the unique band structure.
In the related art, as a device which electrically controls the absorptance of graphene, there is a report of the following device (refer to NPL 2). In the device, graphene is formed on a SiO2 film, a source electrode is formed at one end of the graphene, and a drain electrode is formed at the other end. Then, the SiO2 film, the source electrode and the drain electrode are covered by ion gel, and a gate electrode is formed thereon. The source electrode is grounded and a gate voltage is applied to the gate electrode so that an electric field is applied to the graphene through the ion gel to control the absorptance.
In addition, as a device which electrically controls the absorptance of graphene, there is also a report of a graphene-based waveguide-integrated optical modulator for terahertz optical communication (refer to NPL 3). The graphene-based waveguide-integrated optical modulator has a structure in which graphene is formed on a silicon waveguide with aluminum oxide (Al2O3) interposed therebetween, and controls a degree in which light which is transmitted through the silicon waveguide is absorbed and attenuated by the graphene through application of external voltage. In the graphene-based waveguide-integrated optical modulator, the Fermi level of the graphene is shifted by the voltage application to control the light absorptance of an infrared region.